The camera is configured to take pictures every 20 seconds using CHDK (http://chdk.wikia.com/wiki/CHDK) and one of the stock intervalometer scripts. It is powered by its own rechargeable lithium battery and operates independently of the tracker.

The tracker is based around an atmega328p AVR microcontroller, running code derived from an open source AVR based APRS tracker called the WhereAVR (http://garydion.com/projects/whereavr/). By updating the sine wave generation code to use filtered 7-bit pulse width modulation in place of the 4-bit resistor network, we were able to improve tone quality with a reduced part count. A few additional components were added, including an I2C eeprom for local backup of flight data, external and internal I2C temperature sensors, and a cutdown MOSFET for switching power to a short strip of nichrome wire coiled around the balloon line (the cutdown system was not used in our final flight because we were apprehensive about the complications it added to the balloon rigging).

To keep things light, compact, and simple, we designed a surface mount circuit board to integrate all of the trackers components with a Falcom FSA03 GPS module and a 300mW Radiometrix HX-1 144.39 MHz transmitter. The board was fabricated using the toner transfer method and hand soldered.

For an antenna we used the common 300 ohm twinlead j-pole design (http://www.qsl.net/wb3gck/jpole.htm), tuned as close as possible for use on our frequency (144.39 MHz) and fed with a short length of 50 ohm coax cable for routing/strain relief.

The tracker is powered by a single lithium 9v battery, and regulated down to 5v and 3.3v by linear regulators on the board. The typically undesirable loss of power to heat with these regulators is useful in this case for warming the electronics in the extreme cold temperatures encountered during the flight.

Each chase vehicle had a radio tuned to 146.415 for simplex communication between the vehicles, and another radio tuned to 144.390 for receiving packets from the balloon. For packet decoding we used radios with built in TNCs or laptops with external modems/soundcard modems and TNC software (Soundmodem and Xastir for Linux, AGWPE and UI-View for Windows).